Method and apparatus for transmission measurements to form a 2-d or a 3-d image in tomography applications

ABSTRACT

A method and apparatus for producing radioactive transmission measurements to form a 2-D or a 3-D image with a point source of radiation, such as required in positron emission tomography applications. This involves the passing of the point source proximate the face of a selected each of the tomograph units for the formation of a 3-D image, or a selected portion of the tomograph units for a 2-D image. As such, attenuation data, transmission data, detector performance data, etc., can be obtained. This point source of radiation is rapidly circulated through a conduit that passes across each detector face under the influence of a transport fluid in, for example, an oscillatory motion to achieve a selected radiation field whereby calculation of transmission measurements within a body positioned within the tomograph scanner is achieved. When not being circulated, the radiation source is held within a shield. Circulation of the transport fluid, typically a hydraulic fluid, is typically accomplished using a positive displacement pump. Position sensors are used to monitor the movement of the source in the conduit as well as its position within the shield. Disconnect units permit removal of the radiation source, as contained in the shield, from the system without accessing any other portions of the system.

This application in part discloses and claims subject matter disclosedin my earlier filed pending application, Ser. No. 08/037,303 filed onMar. 26, 1993.

TECHNICAL FIELD

The present invention relates generally to a method and apparatus forthe transmission measurement to form a 3-D image for tomographyapplications, and more particularly to a method and apparatus forrapidly moving a point source of radiation past each detector of thetomograph apparatus together with determining the position of thatsource. Although described specifically for obtaining transmissionattenuation in positron emission tomography, the method and apparatus isapplicable for various purposes in other tomograph devices.

BACKGROUND ART

Positron Emission Tomography (PET) has gained significant popularity innuclear medicine because of the ability to non-invasively studyphysiological processes within the body. PET is the most sensitive, andexhibits the greatest quantification accuracy, of any nuclear medicineimaging instrument available at the present time. Applications requiringthis sensitivity and accuracy include those in the fields of oncology,cardiology and neurology.

Using compounds such as ¹¹ C-labeled glucose, ¹⁸ F-labeled glucose, ¹³N-labeled ammonia and ¹⁵ O-labeled water, PET can be used to study suchphysiological phenomena as blood flow, tissue viability, and in vivobrain neuron activity. These neutron deficient compounds interact withfree electrons in the body area of interest, resulting in theannihilation of the positron. This annihilation yields the emission of apair of photons (gamma rays) approximately 180 degrees (angular) apart.A compound having the desired physiological affect is administered tothe patient, and the radiation resulting from annihilation is detectedby a PET tomograph. After acquiring these annihilation "event pairs" fora period of time, the isotope distribution in a cross section of thebody can be reconstructed.

PET data acquisition occurs by detection of both photons emitted fromthe annihilation of the positron in a coincidence scheme. Due to theapproximate 180 degree angle of departure from the annihilation site,the location of the two detectors registering the "event" define a chordpassing through the location of the annihilation. By histogramming theselines of response (the chords), a "sinogram" is produced that may beused by a process of back-projection to produce a two dimensional imageof the activity. Detection of these lines of activity is performed by acoincidence detection scheme. A valid event line is registered if bothphotons of an annihilation are detected within a coincidence window oftime. Coincidence detection methods ensure (disregarding othersecond-order effects) that an event line is histogrammed only if bothphotons originate from the same positron annihilation.

A recent, likely to become dominant, advance in PET acquisition methodsis the method of data collection referred to as 3-D acquisition. In thetraditional (2-D) acquisition of a modern PET tomograph, an expensive(usually tungsten) collimator known as a septa is placed between theobject within the field-of-view and the discrete axial rings ofdetectors. This septa limits the axial angle that a gamma ray canimpinge on a detector, typically limiting the number of axial rings ofdetectors that a given detector in a specific ring can form acoincidence with to three; one ring toward the front of the tomographfrom the given detector's ring, the same ring that the detector iswithin, and the one ring toward the rear of the tomograph from the givendetector's ring. The methodology of 3-D acquisition removes the septaand allows a given detector to be in coincidence with detectors from allother detector rings. Not only does 3-D acquisition allow removal of thevery expensive septa from the tomograph, but it also affects asignificant increase in tomograph efficiency.

Another tomography diagnostic system is that known as single photonemission computed tomography (SPECT) which is very similar to PET. Thedistinction is that only a single photon from the annihilation withinthe patient is detected. Otherwise, the apparatus is substantially likethat of the PET system.

In computed axial tomography (CAT, or now also referred to as CT), anx-ray source is caused to be passed around a patient. Detectors aroundthe patient then respond to x-ray transmission through the patient toproduce an image of an area of study.

The details of carrying out a PET study are given in numerouspublications. Typically, the following references provide a backgroundfor PET. These are incorporated herein by reference for any of theirteachings.

1. M. E. Phelps, et al.: "Positron Emission Tomography and Audiography",Raven Press, 1986;

2. R. D. Evans: "The Atomic Nucleus", Kreiger, 1955;

3. J. C. Moyers: "A High Performance Detector Electronics System forPositron Emission Tomography", Masters Thesis, University of Tennessee,Knoxville, Tenn., 1990;

4. U.S. Pat. No. 4,743,764 issued to M. E. Casey, et al, on May 10,1988;

5. R. A. DeKemp: "Attenuation Correction in Positron Using Single PhotonTransmission Measurement", Masters Thesis, McMaster University,Hamilton, Ontario, Canada;

6. S. R. Cherry, et al.: "3-D PET Using a Conventional MultisliceTomograph Without Septa", Jl. C. A. T., 15(4) 655-668.

Both SPECT and CAT (or CT) systems are also well known to personsskilled in the art.

In order to achieve maximal quantitative measurement accuracy intomography applications, an attenuation correction must be applied tothe collected emission data. In a PET system, for example, thisattenuation is dependent on both the distance the gamma ray must travelbefore striking the detector, and the density of the attenuating mediain the path of travel. Depending on the location of the annihilationwithin the patient's body, large variations in attenuating media crosssection and density have to be traversed. If not corrected for, thisattenuation causes spatial variant inaccuracies in the images thatdegrade the desired accuracy. As an example, for a cardiac study theattenuation is highest in the line of responses (LORs) passing throughthe width of the torso and arms, and attenuation is lowest in the LORspassing through the front and back of the chest.

Typically, the attenuation correction data in PET systems is produced byeither: shape fitting and linear calculations using known attenuationconstants, these being applicable to symmetric well-defined shapes suchas the head and torso below the thorax (calculated attenuation); orthrough the measurement of the annihilation photon path's attenuationusing a separate transmission scan (measured attenuation). The use ofcalculated attenuation correction, which introduces no statistical noiseinto the emission, can be automated for simple geometries such as thehead, and is the most prominent method used for brain studies. However,complexities in the attenuation media geometry within the chest haveprevented the application of calculated attenuation from being practicalfor studies within this region of the body. Accordingly, transmissionscanning has been utilized.

The total attenuation of a beam along a LOR through an object is equalto the attenuation that occurs for the two photons from an annihilation.Thus, the emission attenuation along the path can be measured by placinga source of gamma rays on the LOR outside of the body and measuringattenuation through the body along this line. It has been the practiceto accomplish this attenuation measurement by placing a cylindricalpositron emitter "sheet" within the PET' field of view (FOV) but outsideof the region (the object) to be measured. By calculating the ratio ofan already acquired blank scan (no object in the FOV) to the acquiredtransmission scan, variations in this ratio data represent the desiredmeasured attenuation. This data is then applied to the emission dataafter a transmission scan of the object to correct for the spacialvariations in attenuation.

There are two types of emitter units conventionally utilized in PETtransmission scan data collection, both of which form a "sheet" ofactivity to surround the patient. One involves the placement of rings ofactivity aligned with detector rings around the inner face of the septa(see FIG. 1). The second type utilizes the rotation of one or moreaxially-oriented rods of activity in a circular path just inside theinner face of the septa (see FIG. 2).

The first of these two emitter systems (the ring source method)significantly reduces the sensitivity of the tomograph due to the closesource-proximity dead time effects of the source activity on all of thedetectors. Further, removal of this assembly is either performedmanually by facility personnel or by a complex automated (more recent)mechanical assembly. Large, cumbersome, out of the FOV shielding isrequired for storage of the automated source when not in use, adding tothe depth of the tomograph tunnel and, thus increasing incidence ofpatient claustrophobia. The second type of emitter, using rotatingsource(s) suffers from the above-mentioned problems and also, due to theshielding requirements, reduces the patient tunnel diameter, furtherincreasing patient claustrophobia symptoms.

Both of the above automated source transportation methods suffer fromhigh mechanical component cost and from low sensitivity. Due to thedead-time-induced reduction in tomograph sensitivity, lengthyacquisitions are required in order to achieve usable low noisetransmission scan data.

Accordingly, it is an object of the present invention to provide amethod and apparatus for rapidly moving a point source of radiationwithin a selected 2-D or 3-D geometry to achieve radiation transmissionmeasurements to form an image within that geometry.

It is also an object of the present invention to provide a system thatreduces the time of determining information in tomography scans.

It is another object of the present invention to provide an improvedradiation emitter for carrying out attenuation data acquisition for usein obtaining increased accuracy in PET scans.

Another object of the present invention is to provide for thecontrolling of a position of a point source of radiation and fordetermining that position so as to generate a 2-D or a 3-D image ofradiation transmission.

It is still another object of the present invention to provide aradiation source of substantially increased activity that can be used intomography applications.

A further object of the present invention is to provide an improvedradiation emitter that requires no mechanical motion within tomographunits, such as the PET unit, but accomplishes emission of radiationuniformly covering all detector coverage in cylindrical regions withinthe unit.

These and other objects of the present invention will become apparentupon a consideration of the drawings forming a part of the disclosure ofthe invention, together with a complete description thereof thatfollows.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, there is provided a method andapparatus for causing a point source of radiation to rapidly move in aselected path around or adjacent an object being subject to a tomographyscan to generate a 3-D image relative to that selected path. This systemutilizes substantially no moving components within the region of theobject and thus substantially reduces cost associated with obtainingtransmission data. The system utilizes a point source carried within atubing placed adjacent the surface of radiation detector faces such thatthe point source is passed adjacent each detector face. This pointsource of a selected shape, which typically can be a sphere or a smallcylinder, is carried within a transport fluid typically moved by apositive displacement pump from a shielded position, through the tubinga selected number of times, and then returned to a storage shield.Typically the source is repetitively passed in an oscillatory mannerthrough the tubing. Thus, the origination of the radiation from thepoint source can be controlled so as to direct radiation across thetomograph apparatus volume and through the object, with the conventionaldetectors being used to determine transmission data. In one embodimentof the invention for use in PET systems, the tubing is formed into acylindrical helix and the transmission data is used to obtain photonattenuation data. In an alternate embodiment, the tubing is formed intoa substantially linear configuration such that the point source is movedin a direction substantially parallel to the longitudinal axis of thetomograph apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric drawing illustrating a typical tomograph unit fora better understanding of the present invention.

FIG. 2 is a cross-sectional view of a typical PET tomograph unitillustrating placement of detectors relative to an object, together withthe placement of the helix of the present invention in the PET unitembodiment.

FIG. 3 is a schematic drawing illustrating the system for producing acylindrical sheet of radiation for attenuation data generation accordingto the one embodiment of the present invention.

FIG. 4 is a schematic drawing showing the circuit whereby signals fromdetectors of tomograph units are processed to determine transmission,etc.

FIG. 5 is a schematic drawing illustrating an alternate embodiment ofthe system for producing a cylindrical sheet of radiation forattenuation data generation according to the one embodiment of thepresent invention.

FIG. 6 is a schematic drawing of the alternate embodiment shown in FIG.5 showing the circuit whereby signals from detectors of tomograph unitsare processed to determine transmission, etc.

FIG. 7 is a schematic drawing of the alternate embodiment shown in FIG.5 showing the impingement of radiation from the point source on a groupof oppositely-disposed detectors.

FIG. 8 is a schematic drawing of the alternate embodiment shown in FIG.5 including a group of collimators for narrowing the impingement of theradiation from the point source to a single oppositely-disposeddetector.

BEST MODE FOR CARRYING OUT THE INVENTION

As discussed hereinafter, the present invention is applicable toproducing 3-D transmission measurements to form an image within thatgeometry. It is especially applicable for many types of tomographyapplications. The invention is described, for purposes of illustration,for a PET unit.

For an understanding of a tomograph unit, and particularly a PET unit,reference is made to FIG. 1 where such unit is indicated generally at10. In general, this unit 10 includes a gantry 12 of conventional designmounted upon a U-shaped mounting bracket 14 supported on a base 16.Detectors 20 for diagnostic imaging operations are carried in acylindrical array on a ring 22, with the faces of the detectors 20forming a cylindrical opening 24 for receiving a selected portion of apatient's body. Signal outputs from the detectors 20 are carried to amonitoring station 26 for analysis and display in a typical manner. Thisstation 26 contains processing means for producing attenuationcorrection data and for combining this attenuation correction data withnormal scan data of the PET unit. The unit includes a patient bed 28,which includes a sliding carriage 30, for moving the selected bodyportion into and out of the opening 24 in a conventional manner.

FIG. 2 is a cross-sectional view of the aforementioned ring 22 with thedetector units 20 mounted thereon. This figure illustrates the positionof a patient 23 as located on a central axis of the cylindrical array ofdetectors 20. This geometric arrangement is such as to generate thepositron radiation from within the patient to impinge upon the variousdetectors 20 for achieving PET scan data. The prior devices forachieving attenuation data (rings of radiation or rotating radiationsources) were placed adjacent this cylindrical surface defined by thedetectors 20. This is the same location, as indicated at 32, for thelocation of the present invention when applied to a PET unit.

A schematic diagram of the present invention, as adapted for PET-typetomography, is shown at 34 in FIG. 3. A helix 36 formed from a suitablesubstantially rigid tubing forms a cylindrical unit dimensioned to bereceived in the location 32 of FIG. 2. This tubing can be "potted" witha suitable material (e.g., a low radiation attenuation plastic) into asolid structure so as to maintain its geometry. The interior dimensionof the tubing of the helix is chosen to receive a point source radiationemitter unit 38. The desired source of radiation (e.g., gamma rays) canbe, for example, either a small sphere or a small cylinder. It will beunderstood that the interior shape of the tubing is chosen to becompatible with the configuration of the point source of radiation. Thespiral is typically formed from polyvinyl chloride (PVC) tubing orsimilar material exhibiting low attenuation to the selected radiation,and has an interior diameter typically about 3 mm. The external diameterof the tubing, and the pitch of the helix, is chosen so that the pointsource when within a location within the tubing passes past anindividual one of the detector faces, with each detector face "seeing"the point source. The helix diameter is typically about 90 cm which isthe diameter of the PET system inside the faces of the detectors.Typically the source 38 is ⁶⁸ Ge encapsulated in a gold enclosure whichproduces positrons like those that are emitted during the annihilationwithin the patient's body. Since gold is rather easily abraded, an outerhard coating of plastic or other low attenuation material is usuallyapplied over the gold layer. Of course, it will be understood that othertypes of radiation, and sources for that radiation, can be utilized inthe present invention.

This point source emitter unit is carried through the system in asuitable transport fluid. Preferably, this transport fluid issubstantially non-compressible and thus is a liquid such as ethyleneglycol or other low attenuation liquids. The transport fluid is movedtypically by a positive displacement pump 40 such as a peristaltic pump.Other substantially non-compressible low attenuation fluids that areresistant to radiation damage will be known to persons skilled in theart. This pump 40 is connected into a fluid circuit that includes tubing41 and tubing 42 that leads from a source shield 44 to the entrance ofthe helix 36. Of course, for a peristaltic pump there is a short sectionof flexible tubing, as at 43A, for passage through the pump 40.Typically, the tubing sections are polyvinyl chloride (PVC). Further,other means of moving the transport fluid through the helix 36 will beknown to persons skilled in the art.

The source shield 44 is formed from a high-Z material (e.g., lead) andcompletely surrounds the path of the source 38 therein. This shield 44is preferably arranged such that, by using the disconnects 46, 48, theshield 44 containing the source 38 can be removed from the system.Further, the specific gravity of the source 38 is chosen to be slightlyhigher than that of the transport fluid such that the source 38 will beretained in the "loop" 50 formed within the shield 44, when desired, inthe absence of transport fluid flow. In addition, a mechanical "stop" 52is preferably located within the shield 44 to prevent inadvertentdischarge of the source 38 therefrom. This arrangement thus requiresfluid circulation only during desired movement of the source 38.

In order to ascertain locations of the source 38 during movement orduring positioning within the shield 44, the present system is typicallyprovided with position sensors: sensor 54 is located at the entrance tothe helix 36; sensor 56 is located at the exit from the helix 36; andsensor 58 is located within the shield 44 at the "park" position. Thesecan be, typically, optical sensors when the tubing of the helix 36 issubstantially transparent. For example, sensors 54 and 56 determine whenthe source 38 enters and reaches the exit of the helix 36, respectively.This prevents leaving the source 38 stationary within the helix 36. Thensensor 58 ascertains that the source 38 has actually been returned tothe shield 44.

Whenever attenuation data is desired, the mechanical stop 52 iswithdrawn and operation of the pump 40 is initiated in a direction tocause the source 38 to be moved toward the entrance of the helix 36.This causes the source 38 to be withdrawn from the shield 44 andcirculated through the helix 36 via tubing 42. The sensors 54, 56 ensurethat complete passage through the helix 36 has occurred. Typically logiccircuitry 59 associated with sensors 54, 56 causes reversal of the pump40 forcing the source 38 in a reverse direction until detected by sensor54. Again there is a reversal, with this oscillatory movement continuingfor a selected number of times to assure desired statistical data. Thesource 38 then is returned to the shield 44 through tubing 42 due tocontinued operation of the pump 40 until it again reaches the mechanicalstop 52, with the location being ascertained by sensor 58.

During the circulation of the source 38 through the helix 36, theconventional PET radiation detectors 20 (see FIGS. 1 and 2) record thereceived radiation, and these data are processed in a conventionalsignal processor known to those skilled in the art to obtain theattenuation data that then can be used by a signal processor toappropriately adjust scan data received on the basis of annihilationevents within the patient's body. As stated above, this processingoccurs within circuitry at the station 26.

A schematic drawing depicting the receiving of radiation at detectors,and the processing of the signals, is contained in FIG. 4. In thisdrawing, several detectors 20 (of a multiplicity of detectors) onoppositely-disposed sides of the tomograph unit are depicted: it will beunderstood that there are many other detectors in a normal system. Thetubing 42 passes the face of each of the detectors such that the pointsource of radiation 38 can pass over the detector face. Photon radiation60, 60A (two photons as in the PET system) emanate from a point, P, ofpositron annihilation within a patient 23 in opposite directions towardthe detectors 20. Further, photon radiation 62 from the point sourcepasses through the patient 23, typically in the direction shown when thesource is in the tubing at the left in the drawing. Signals from all ofthe detectors 20 are processed in pre-programmed analyzers 64, with theresult being stored in storage unit 66 and depicted on a display unit68. The processing of signals, their storage and their display areaccording to technology that will be known to those versed in the art.

A schematic diagram of an alternate embodiment of the present inventionis shown at 34' in FIG. 5. In lieu of the helix 36 as formed in theembodiment illustrated in FIGS. 3 and 4, the tubing is configured toform a linear portion 70 between the sensors 54', 56'. The linearportion 70 may be secured within the PET system proximate one row ofdetectors 20 in a direction parallel to the longitudinal axis of thering 22. It is envisioned that the linear portion 70 may be securedproximate any one row of detectors 20, and further may be moved from onerow of detectors 20 to another in order to allow the point source 38' topass each detector face.

As in the previously described embodiment, whenever attenuation data isdesired, the mechanical stop 52' is withdrawn and operation of the pump40' is initiated in a direction to cause the source 38' to be movedtoward the entrance of the linear portion 70. The sensors 54',56' ensurethat complete passage through the linear portion 70 has occurred. Thelogic circuitry 59' associated with sensors 54' 56' causes reversal ofthe pump 40' forcing the source 38' in a reverse direction untildetected by sensor 54'. Again there is a reversal, with this oscillatorymovement, as indicated by arrow 72, continuing for a selected number oftimes to assure desired statistical data. It is envisioned that, in anembodiment where the linear portion 70 is moved from one row ofdetectors 20 to another, a electromechanical device (not shown) may beincorporated to cause such movement. The electromechanical device may beactuated by the logic circuitry 59' such that after a selected number ofoscillations of the point source 38' within the linear portion 70, thelinear portion 70 may be moved to an adjacent or other selected row ofdetectors 20.

A schematic drawing depicting the receiving of radiation at detectors inthe alternate embodiment, and the processing of the signals, iscontained in FIG. 6. The tubing 42' passes the face of each of one rowof the detectors 20 such that the point source of radiation 38' can passover each detector face in that row. Photon radiation 60, 60A (twophotons as in the PET system) emanate from a point, P, of positronannihilation within a patient 23 in opposite directions toward thedetectors 20. However, in this embodiment, only the photon radiation 60Adirected toward the detector 20 opposite the linear portion 70 isconsidered. Further, photon radiation 62 from the point source passesthrough the patient 23. Signals from the detectors 20 are processed inpre-programmed analyzers 64, with the result being stored in storageunit 66 and depicted on a display unit 68.

As shown in FIG. 7, the radiation from the point source 38 is actuallydirected in a broad range of directions such that a plurality ofdetectors 20 are impinged upon. In this Figure, the tubing within whichthe point source 38 is passed has been omitted, as either configurationof tubing may be used. The point source 38 is depicted as beingpositioned in front of the detector 20A. In this case, the detector 20Ddirectly across from the detector 20A which is being passed by the pointsource 38 will receive the greatest magnitude of radiation. Asillustrated, with the point source 38 positioned in front of anydetector 20, every other detector 20 will receive an amount of radiationdependent upon the distance away from the point source 38 and theportion of the human body 23 through which it passes. Thus,2-dimensional data may be derived between any two detectors 20positioned colinearly with the point source 38. A compilation of all ofthe 2-dimensional data yields a 3-dimensional image.

In an alternate embodiment as shown in FIG. 8, a plurality ofcollimators 74 may be placed to separate the individual rings ofdetectors 20 such that the radiation produced by the point source 38 isdirected solely toward the detectors 20 in that same ring and theimmediately adjacent rings. For example, as illustrated when the pointsource is in front of the detector 20A, only those detectors 20D,E areable to receive a signal. When the point source 38 is in front of thedetector 20B, each of the detectors 20D,E,F receive a signal. Thus, aseries of 2-dimensional images may be aquired, but all of the images aresubstantially parallel with each other. Due to the inability to developa 2-dimensional image between detectors 20 in rings more than one ringaway, 3-dimensional imaging is not possible. The basic advantage ofproducing a 2-dimensional image as opposed to a 3-dimensional image isthat a 2-dimensional image is much less time consuming and, thus, lessexpensive. However, with advances that are being made in thecomputational processing time, the derivation of 3-dimensional images isbecomning more readily available.

The approach to acquiring transmission attenuation data using thepresent invention involves using a point source transmission andacquiring the data in a "singles" mode. Instead of using coincidences,as used in the prior art, to determine the LOR chords, the chords aredescribed by the points of the detected event determined by detectorsopposite the patient from the source, and the location of the source.Because of the sensors (e.g., 54,56), the position of the source can beaccurately located. Then, given the location of these two points in athree dimension space, the chord is accurately described. By collectingdata in this mode, the detector system is not paralyzed by the dead timelosses of the detector adjacent to the transmission source as is thecase when requiring transmission data in the prior coincidence method.The configuration, as described, not only provides an acquisition systemmore sensitive for a given amount of radiation activity than using thecoincidence system, but permits using a radiation source having asignificantly increased specific activity with a resulting increase inacquired counts. This results in substantially reduced acquisition time.This increase in activity is permitted since detectors adjacent thesource, which will be paralyzed by the activity, are not used forestablishing an end of the LOR chords as in the prior art.

From the foregoing, it will be understood by persons skilled in the artthat an improvement has been made to the manner of determiningattenuation data in a positron emission tomograph unit. Further, whileproviding data of attenuation through a body for PET scanning, thesystem can be used to determine overall response of radiation detectorsof the basic PET system.

The present invention has been described in detail as applied topositron emission tomograph (PET) units for illustration purposes. Dueto the ability of the method and apparatus to rapidly move a pointsource of radiation in a selected 2-D or 3-D geometry, and to determinethe position of that source, the present invention is applicable tovarious tomography applications. Further, the present invention isapplicable for producing 2-D and 3-D transmission measurement images inany selected geometry and for any desired utilization of such images.

Although certain specific materials are recited herein, these are forillustrative purposes and not for limiting the invention. Accordingly,the invention is to be limited only by the appended claims andequivalents thereof when read together with the complete description ofthe present invention.

We claim:
 1. A method for forming multi-dimensional attenuationcorrection data of radiation transmission through an object positionedin a tomograph device, said device having radiation detectors defining aplurality of faces of said radiation detectors, said methodcomprising:positioning a selected point source of radioactive radiationproximate each of said radiation detectors in a selected sequence usinga selected transporting device, said radioactive radiation beingdirected through the object and received by said radiation detectors ofsaid tomograph device; and processing signals received from outputs ofsaid detectors of said tomograph device to determine transmission dataof radiation from said source of radiation during passage of saidradiation through the object for forming said attenuation correctiondata.
 2. The method of claim 1 further comprising detecting the locationof said source of radiation.
 3. The method of claim 1 wherein saidtomograph device includes a plurality of collimators positioned betweeneach consecutive pair of rings comprised of said radiation detectors,said multi-dimensional attenuation correction data beingtwo-dimensional.
 4. The Method of claim 1 wherein lines of responsedefined between pairs of said radiation detectors in uncommon ringscomprised of said radiation detectors are unobstructed, saidmulti-dimensional attenuation correction data being three-dimensional.5. A method for forming a multi-dimensional image of radiationtransmission within an object positioned in a tomograph device, saiddevice having radiation detectors with a means to effect a particularmulti-dimensional image defining a plurality of faces of said radiationdetectors, said method comprising:forming a conduit from tubing, saidtubing having a selected interior diameter, said conduit having aconfiguration so as to have a portion positioned adjacent each of saidplurality of radiation detector faces, said conduit having an inlet andan outlet; filling said conduit with a transport fluid; pumping saidtransport fluid within said conduit with a pump, said pump having aninlet and an outlet, said pump inlet being in fluid communication withsaid outlet of said conduit; positioning a selected point source ofradioactive radiation for a particular multi-dimensional use within saidtubing of said conduit for circulating through said conduit bycirculating said transport fluid, said radioactive radiation beingdirected through the object and received by said detectors of saidtomograph device; storing said point source of radiation within astorage shield when not being circulated through said conduit, saidstorage shield having an outlet in fluid communication with said inletof said conduit and an inlet in fluid communication with said outlet ofsaid pump; and processing signals received from outputs of saiddetectors of said tomograph device to determine for a particularmulti-dimensional use transmission data of radiation from said source ofradiation during passage of said radiation through the object.
 6. Themethod of claim 5 further comprising detecting the location of saidsource of radiation within said conduit and within said shield.
 7. Themethod of claim 5 wherein said tomograph device includes a plurality ofcollimators positioned between each consecutive pair of rings comprisedof said radiation detectors, said multi-dimensional image being atwo-dimensional image.
 8. The Method of claim 5 wherein lines ofresponse defined between pairs of said radiation detectors in uncommonrings comprised of said radiation detectors are unobstructed, saidmulti-dimensional image being a three-dimensional image.
 9. A system forproducing a selected distribution of radioactive radiation within aselected multi-dimensional imaging device, said device having radiationdetectors defining a plurality of faces of said radiation detectors withmeans to effect detecting a multi-dimensional image, said systemcomprising:a conduit formed into a selected configuration correspondingto said selected distribution, said conduit having a passageway of aselected cross-sectional configuration, said conduit having an inlet andan outlet; a transport fluid contained within said conduit; a pump forcirculating said transport fluid within said passageway of said conduit,said pump having an inlet and an outlet, said pump outlet being in fluidcommunication with said inlet to said conduit; and a selected source ofradioactivity within said conduit for being circulated within saidconduit by circulation of said transport fluid.
 10. The system of claim9 further comprising a storage shield for containing said source ofradioactivity when not being circulated through said conduit, saidstorage shield having an inlet in fluid communication with said inlet ofsaid conduit and an outlet in fluid communication with said outlet ofsaid pump.
 11. The system of claim 9 further comprising first and seconddetection units positioned proximate selected locations along saidconduit to monitor for a presence of said source of radioactivity withinsaid conduit between said first and second detection units.
 12. Thesystem of claim 11 wherein said detection units are connected to saidpump to repetitively reverse pumping directions of said transport fluidwhereby said source of radioactivity moves in an oscillatory directionwithin said conduit between locations of said first and second detectionunits.
 13. The system of claim 9 wherein said selected cross-sectionalconfiguration of said passageway is circular, and wherein said source ofradioactivity is spherical and has a diameter to be closely receivedwithin said passageway of said conduit.
 14. The system of claim 9wherein said source of radioactivity is cylindrical and has a diameterto be closely received within said passageway of said conduit.
 15. Thesystem of claim 9 wherein said pump is a peristaltic pump forcirculating said transport fluid through said conduit.
 16. The system ofclaim 11 further comprising a further detection unit positioned at saidstorage shield to ascertain presence of said source of radioactivitywithin said storage shield.
 17. The system of claim 16 furthercomprising a physical stop element within said storage shield proximatesaid further detection unit to selectively hold said source ofradioactivity within said storage shield.
 18. The system of claim 10further comprising disconnect units at said inlet and outlet of saidstorage shield whereby said storage shield containing said source ofradioactivity can be disconnected from said conduit and said pump. 19.The system of claim 9 wherein said selected distribution of saidradioactive radiation is three-dimensional wherein lines of response aredefined between pairs of said radiation detectors disposed in uncommonrings comprised of said radiation detectors are substantiallyunobstructed.
 20. The system of claim 9 wherein said selecteddistribution of said radioactive radiation is two-dimensional andwherein said system further includes a plurality of collimators betweenrespective pairs of rings comprised of said radiation detectors.
 21. Asystem for forming a multi-dimensional image of radiation transmissionwithin an object positioned in a tomograph device, said device havingradiation detectors with a means to effect a particularmulti-dimensional image defining a plurality of faces of said radiationdetectors, said system comprising:a conduit formed from cylindricaltubing, said tubing having a selected interior configuration, saidconduit having an external configuration so as to have a portionpositioned adjacent at least a selected portion of said plurality ofradiation detector faces, said conduit having an inlet and an outlet; atransport fluid contained within said conduit; a pump for circulatingsaid transport fluid within said conduit, said pump having an inlet andan outlet, said pump inlet being in fluid communication with said outletof said conduit; a selected point source of radioactive radiationpositioned for a particular multi-dimensional use within said tubing forcirculating through said conduit by circulation of said transport fluid,said radiation directed through the object and received by saiddetectors of said tomograph device; storage shield for containing saidsource of radioactivity when not being circulated through said conduit,said storage shield having an inlet in fluid communication with saidoutlet of said conduit and an outlet in fluid communication with saidinlet to said pump; and signal processing circuitry connected to outputsof said detectors of said tomograph device to determine for a particularmulti-dimensional use transmission data of radiation from said source ofradiation during passage of said radiation through the object.
 22. Thesystem of claim 21 further comprising further processing circuitryconnected to said outputs of said detectors of said tomograph device andto said signal processing circuitry to apply said transmission data toof said radiation source to data of annihilation photon transmissionfrom within the object to said detectors of said tomograph device. 23.The system of claim 21 further comprising first and second detectionunits positioned proximate selected locations along said conduit tomonitor for a presence of said source of radiation within said conduitbetween said selected locations.
 24. The system of claim 23 whereinsensors of said detection units are connected to said pump torepetitively reverse pumping directions of said transport fluid wherebysaid source of radiation moves in an oscillatory direction within saidconduit between said selected locations of said first and seconddetection units.
 25. The system of claim 21 wherein said source ofradiation is spherical and has a diameter to be closely received withinsaid interior configuration of said tubing of said conduit.
 26. Thesystem of claim 21 wherein said source of radiation is cylindrical andhas a diameter to be closely received within said interior configurationof said tubing of said conduit.
 27. The system of claim 21 wherein saidpump is a peristaltic pump for circulating said transport fluid throughsaid conduit.
 28. The system of claim 23 further comprising a furtherdetection unit positioned at said storage shield to ascertain presenceof said source of radiation within said storage shield.
 29. The systemof claim 28 further comprising a physical stop within said storageshield proximate said further detection unit to selectively hold saidsource of radiation within said storage shield in the absence ofoperation of said pump.
 30. The system of claim 21 further comprisingdisconnect units at said inlet and outlet of said storage shield wherebysaid storage shield means containing said source of radiation can bedisconnected from said conduit and said pump.
 31. The system of claim 21wherein said transport fluid is a hydraulic fluid having a specificgravity less than that of said source of radiation.
 32. The system ofclaim 21 wherein said multi-dimensional image of said radiationtransmission is three-dimensional wherein lines of response are definedbetween pairs of said radiation detectors disposed in uncommon ringscomprised of said radiation detectors are substantially unobstructed.33. The system of claim 21 wherein said multi-dimensional image of saidradiation transmission is two-dimensional and wherein said systemfurther includes a plurality of collimators between respective pairs ofrings comprised of said radiation detectors.
 34. A system for forming amulti-dimensional image of photon attenuation within an objectpositioned in a positron emission tomograph device, said device havingradiation detectors with a means to effect a particularmulti-dimensional image defining a cylindrical surface around theobject, said system comprising:a helix formed from cylindrical tubing,said tubing having a selected interior configuration helix, said helixhaving a selected exterior diameter and a selected pitch and positionedadjacent said cylindrical surface defined by said radiation detectorswith a portion of each loop of said helix adjacent a face of each one ofsaid radiation detectors, said helix having an inlet and an outlet; ahydraulic fluid contained within said helix, said hydraulic fluid havinga selected specific gravity; a pump for circulating said hydraulic fluidwithin said helix, said pump having an inlet and an outlet, said pumpoutlet being in fluid communication with said inlet to said helix; aselected source of radiation of a selected configuration for aparticular multi-dimensional use for circulating through said helix bycirculation of said hydraulic fluid, said radiation passing through theobject to be attenuated by the object and received by said radiationdetectors of said positron emission tomograph device; a storage shieldfor containing said source of radiation when not being circulatedthrough said helix, said storage shield having an outlet in fluidcommunication with said inlet of said helix and an inlet in fluidcommunication with said outlet to said pump, said inlet and outlet ofsaid storage shield having disconnect units whereby said storage shieldcontaining said source of radiation can be disconnected from said helixand said pump; detection units containing sensors positioned proximatesaid inlet and said outlet of said helix to monitor for a presence ofsaid source of radiation entering and reaching said outlet of saidhelix, said sensors being electrically connected to said pump torepetitively reverse pumping directions of said hydraulic fluid wherebysaid source of radiation moves in an oscillatory direction within saidhelix for a selected time; a physical stop within said storage shield toselectively hold said source of radiation within said storage shield inthe absence of pump operation; a further detection unit positionedwithin said storage shield proximate said stop to ascertain presence ofsaid source of radiation within said storage shield; signal processingcircuitry connected to outputs of said detectors of said positronemission tomograph device to determine for a particularmulti-dimensional use attenuation data of photons from said source ofphotons during passage through the object; and further processingcircuitry connected to said outputs of said detectors of said positronemission tomograph device and to said signal processing circuitry toapply said attenuation data to data of annihilation photon transmissionfrom within the object to said detectors of said positron emissiontomograph device.
 35. The system of claim 34 wherein said source ofradiation is ⁶⁸ Ge encapsulated in a cladding of low attenuation,wherein said selected specific gravity of said hydraulic fluid is lessthan a specific gravity of said source of photons, and wherein saidtubing is polyvinyl chloride.
 36. The system of claim 34 wherein saidmulti-dimensional image of said photon attenuation is three-dimensionalwherein lines of response are defined between pairs of said radiationdetectors disposed in uncommon rings comprised of said radiationdetectors are substantially unobstructed.
 37. The system of claim 34wherein said multi-dimensional image of said photon attenuation istwo-dimensional and wherein said system further includes a plurality ofcollimators between respective pairs of rings comprised of saidradiation detectors.